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TPS61021A: When configured as current source, operating mode switches back and forth and current is erratic

lslonim
Expert 1710 points
Part Number: TPS61021A

Hi,

I am working on a project for a client, using the TPS61021A. I’m using it in a topology where the TPS61021A is operating as a current source rather than a voltage source, see sketch. Sorry there is an error in the sketch, the inductor is 0.47uH which is the suggested value for this part. I can’t describe the circuit or the load in detail on a public forum, but a 3V Zener is a reasonable approximation of what the load is. The output current is 2A, which is to say when 2A is flowing through the sense resistor, the feedback voltage to the switcher is 790mV. The supply voltage to the system starts at 3.2V and over a few minutes drops to 2V. For the purposes of this test I set the supply voltage to a fixed value of 2.9V, which is in the zone where my problem is. Above 3.1V and below 2V, the problem does not happen.

The load current is erratic, see scope shot. The blue trace is the voltage to the Zener, and the yellow is the feedback voltage, which is at 790mV, right where it should be. The chip seems to be switching between two types of operation – one with small sinusoidal variations in voltage which do not cause a problem, and one with longer off times which vary quite a bit. The longer off times produce larger swings in current and cause problems in the product.

Can anyone give me some guidance as to what is going on inside the TPS61021, why the switching modes seem to change, and how I can keep it in the more well-behaved operating mode? I thought it might be due to the holdup caps on the output being too large and having the chip think there was no load, but changing from 44uF to 22uF didn’t make a difference in the problem.

Thanks for any suggestions,

Lloyd

  • 0
    TI__Mastermind 34231 points

    Hi Lloyd,

    It's a interesting application that I never met before.

    May I know where is FB pin connected? What does X50 represent?

  • 0
    TI__Mastermind 46905 points

    Hi LIoyd,

    it seem the circuit is unstable. did you simulate the circuit in TINA or PSIPICE to check the stability. 

    the 4.7uH inductor seem to be too large. could you try 1uH or 0.47uH.

  • 0 in reply to Zack Liu
    Expert 1710 points

    Zack,

    The block showing X50 is an analog gain of 50. The voltage developed across the 8milliOhm current sense resistor is multiplied by 50 which gives about 800mV with 2A, which is the goal. In the real system there is an offset added to the multiplier to allow me to change the current setting, but that is not shown here and its value is fixed during these tests.

    What I am trying to do is have a variable current source that can put out 0-2A into 3V with a supply that varies from 2V-3V. So I chose a boost switcher and made it be a current source. The control is an analog voltage that offsets the output of the current sense gain stage. I suspect that the phase shift/time delay through the analog gain stage may be causing instability but that's beyond my expertise. I'd love to find a solution that is designed for a case like this if you have any suggestions. 

    Lloyd

  • 0 in reply to Jasper Li
    Expert 1710 points

    Jasper,

    I'm very sorry, that was a mistake on my part, the inductor is 0.47uH which is what is suggested for this part.

    I downloaded TINA and started a tutorial but it seems like it will take me a long time to understand how to use it. And since the conditions to cause this problem involve adjusting the supply voltage up and down somewhat at random till it happens, I think I will ask for the opinion of the experts before I put in that time.

  • 0 in reply to lslonim
    TI__Mastermind 46905 points

    Hi Islonim,

    I'm not sure which experts do you refer to. I'm the application engineer from boost converter product line. and this device is manly supported by me and Zack.

    the control loop of the circuit is big difference with the typical application condition of the device. and the bandwidth of the amplifier also impact a lot on the stability. the 1K and 0.1uF RC filter also impact the stability. that is why I suggest we should simulate it firstly. TINA is not hard to use. please try it.

    unfortunately,  resource is not allowed. otherwise, I can do the simulate for you. sorry about this.

  • 0 in reply to Jasper Li
    Expert 1710 points

    Jasper,

    Of course the experts I am referring to are you and Zack! I am very grateful for your input.

    Thanks for the suggestion of TINA. I had downloaded TI's version of PSPICE and it looks very complicated. I will try TINA.

    Maybe my whole approach is wrong - is there a better way to do a controlled current source that will work in my application? I can't go into the details on a public forum, but can I send you a direct message giving you more background?

    Thanks,

    Lloyd

  • 0 in reply to lslonim
    TI__Mastermind 46905 points

    Hi Lloyd,

    Your circuit is similar as the Figure 4 of this application note https://www.ti.com/lit/an/slva419c/slva419c.pdf 

    i think such circuit should work. 

  • 0 in reply to Jasper Li
    Expert 1710 points

    Thanks, I will try that.

    Lloyd